The invention relates to the general field of communications.
The invention relates more particularly to a method of designating a group of transmitter-receiver pairs authorized to communicate simultaneously over a telecommunications network when interference alignment techniques are implemented on the network in order to manage the multiuser interference that is generated by such simultaneous communications. Such a method is also known as a clustering (or grouping) method.
A preferred but non-limiting application of the invention lies in the field of wireless networks, such as for example wireless fidelity (WiFi®) networks in which a plurality of access points may be deployed to serve a plurality of users, or mobile networks using femtocells, as defined by the third generation partnership project (3GPP) standard.
In known manner, a major problem encountered in networks in which a plurality of transmitter-receiver pairs may be communicating simultaneously lies in the interference these transmitter-receiver pairs generate between one another.
Various techniques have been proposed to manage such interference.
In such techniques, so-called interference alignment techniques (or algorithms) consist:                firstly in confining the interference seen by a particular receiver in a vector subspace of small dimension, by applying a precoding matrix to the useful signal at the transmitter paired with the receiver; and        secondly in using the remaining dimensions for decoding the useful signal by projecting it onto a vector subspace orthogonal to the vector subspace associated with the interference.        
In systems comprising a plurality of transmitter-receiver pairs respectively having multiple transmit antennas and multiple receive antennas (also known as multiple-input multiple-output (MIMO) systems), such interference alignment techniques also make use of the space dimension made available by the multiple transmit and receive antennas in order to achieve alignment of interference.
For MIMO systems, there exist in particular interference alignment techniques that are iterative. Such techniques present the advantage of not requiring overall knowledge at each transmitter about the propagation channels associated with all of the transmitter-receiver pairs participating in the interference alignment. More precisely, they make use of signal precoding at the transmitter (e.g. a device A) and elimination of interference at the receiver (e.g. a device B) by using filters, which filters are updated on each iteration by alternating between transmitting over the direct network (transmission from A to B) and transmitting over the dual network (transmission form B to A). For this purpose, certain iterative techniques rely in particular on channel reciprocity between the direct network and the dual network.
In general manner, the feasibility of interference alignment depends on parameters of the system under consideration. Thus, for a MIMO system, the feasibility of interference alignment depends in particular on the number of transmitter-receiver pairs attempting to communicate simultaneously, on the number of transmit antennas and the number of receive antennas of the transmitter-receiver pairs (which numbers are not necessarily identical for each of the pairs), and on the number of parallel data streams exchanged between each transmitter-receiver pair. In other words, depending on the parameters of the system, interference alignment may be achieved for only a limited number NG≦NGmax of transmitter-receiver pairs that are communicating simultaneously.
Given this constraint, it is possible to implement grouping methods in order to designate a group of NG transmitter-receiver pairs suitable for communicating simultaneously over the network and for executing an interference alignment technique effectively (i.e. for achieving interference alignment).
By way of example, the NG transmitter-receiver pairs making up the group may be designed randomly. Nevertheless, it will readily be understood that even though opting for grouping in this way does not add complexity to the system, it does not enable system performance to be guaranteed, in particular in terms of data rate.
In the present state of the art there exist alternative grouping methods that enable the data rate achieved by the system to be improved and that consist in selecting a group of transmitter-receiver pairs that satisfy determined criteria. One such method is described in particular in Document U.S. Pat. No. 8,036,098.
That method, shown diagrammatically in FIG. 1, sets a minimum target data rate R to be achieved and a group size NG enabling interference alignment to be performed (step E10).
Furthermore, during a first stage I, transmitter-receiver pairs are identified for which the matrices of the direct channels present singular values that exceed a predetermined threshold T (step E20). The NG transmitter-receiver pairs having the greatest singular values are then selected to form a group in order to align interference (step E30). The total data rate made available by the NG selected pairs is calculated (step E40) and compared with the target data rate R (step E50).
If the total data rate exceeds the target data rate R (response yes to the test of step E50), the group of NG selected pairs is validated and it executes an interference alignment technique (step E90).
Otherwise, in contrast, a second stage II begins of searching for a new group.
During this second stage II, a group of NG+c transmitter-receiver pairs is selected randomly (step E60). Thereafter, for each possible subgroup of NG transmitter-receiver pairs selected from this group, the total data rate that can be achieved is evaluated while taking account of an application of a non-iterative interference alignment technique that requires knowledge about the propagation channel at the transmitter (step E70). The subgroup of NG pairs corresponding to the greatest total data rate is selected (step E80) in order to execute the interference alignment algorithm (step E90).
Nevertheless, although such a method does indeed make it possible to improve the data rate that is achieved by the system, it does not take account of the total spectrum efficiency of the system.
It may also present a large amount of complexity when the exhaustive search stage II is performed, in particular because of repeated execution of the interference alignment technique and because of the information required for such execution (e.g. global knowledge of the propagation channel at each transmitter). Such complexity increases even further when consideration is given to an iterative interference alignment technique, since that generally requires a large number of iterations in order to converge.
One of the objects of the invention is to remedy the insufficiencies of grouping methods in the state of the art.